The thermal conductivity $\kappa$ of the quasi-2D organic spin-liquid candidate ${\mathrm{EtMe}}_3\mathrm{Sb}[\mathrm{Pd}(\mathrm{dmit}{)}_2{]}_2$ (dmit-131) was measured at low temperatures, down to 0.07 K. We observe a vanishingly small residual linear term ${\kappa }_0/T$, in $\kappa /T$ vs $T$ as $T\to 0$. This shows that the low-energy excitations responsible for the sizable residual linear term $\gamma$ in the specific heat $C$, seen in $C/T$ vs $T$ as $T\to 0$, are localized. We conclude that there are no mobile gapless excitations in this spin-liquid candidate, in contrast with a prior study of dmit-131 that reported a large ${\kappa }_0/T$ value [ , Science 328, 1246 (2010)]. Our study shows that dmit-131 is in fact similar to $\kappa \text{−}(\mathrm{BEDT}\text{−}\mathrm{TTF}{)}_2{\mathrm{Cu}}_2(\mathrm{CN}{)}_3$, another quasi-2D organic spin-liquid candidate where a vanishingly small ${\kappa }_0/T$ and a sizable $\gamma$ are seen. We attribute heat conduction in these organic insulators without magnetic order to phonons undergoing strong spin-phonon scattering, as observed in several other spin-liquid materials.

The centrosymmetric tetragonal itinerant magnet ${\mathrm{EuAl}}_4$ exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of ${\mathrm{EuAl}}_4$ by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess $\sim 0.10$% and $\sim 0.03$% orthorhombic structural distortion within the $ab$ plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in ${\mathrm{EuAl}}_4$, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.

A quasi-one-dimensional organic charge-transfer salt ${(\mathrm{TMTTF})}_2{\mathrm{PF}}_6$ undergoes a multistep phase transition as the temperature decreases. One of these transitions is called a “structureless transition,” and these detailed structures were unknown for many years. With synchrotron x-ray diffraction, we observed a slight structural difference owing to the effect of charge-order transition between two TMTTF molecules in a dimer, which corresponds to the charge transfer ${\delta }_{\mathrm{CO}}=0.20e$. The two-dimensional Wigner crystallization was determined from an electron density analysis using core differential Fourier synthesis. Furthermore, we found that the ground state due to tetramerization, called the spin Peierls phase, is a three-dimensional transition with interchain correlation.

We investigated the precise crystal structures and electronic states of a quasi-two-dimensional molecular conductor $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ at ambient pressure. The electronic resistivity of this molecular solid shows metal-to-insulator (MI) crossover behavior at $T_{\mathrm{MI}}=50\mathrm{K}$. Our x-ray diffraction and ${}^{13}\mathrm{C}$ nuclear magnetic resonance experiments revealed that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ maintains the inversion symmetry below $T_{\mathrm{MI}}$. First-principles calculations found a pair of anisotropic Dirac cones at a general $k$ point, with the degenerate contact points at the Fermi level. The origin of the insulating state in this system is a small energy gap of $\sim 2\mathrm{meV}$ opened by the spin-orbit interaction. The $Z_2$ topological invariants indicate that this system is a weak topological insulator. Our results suggest that $\alpha \text{−}{\left(\mathrm{BETS}\right)}_2{\mathrm{I}}_3$ is a promising material for studying the bulk Dirac electron system in two dimensions.

$1T-\mathrm{TiS}{\mathrm{e}}_2$ has a semimetallic band structure at room temperature and undergoes phase transition to a triple-$q$ charge-density wave (CDW) state with a commensurate superlattice structure $\left(2a\times 2a\times 2c\right)$ below $T_c\approx 200\mathrm{K}$ at ambient pressure. This phase transition is caused by cooperative phenomena involving electron-phonon and electron-hole (excitonic) interactions, and cannot be described by a standard CDW framework. By Cu intercalation or the application of pressure, this phase-transition temperature is suppressed and superconductivity (SC) appears. However, it is not clear what kind of order parameters are affected by these two procedures. We investigated the crystal structure of $\mathrm{C}{\mathrm{u}}_x\mathrm{TiS}{\mathrm{e}}_2$ and pressurized $1T-\mathrm{TiS}{\mathrm{e}}_2$ around the SC state by synchrotron x-ray diffraction on single crystals. In the high-temperature phase, the variations of structural parameters for the case of Cu intercalation and application of pressure are considerably different. Moreover, the relationship between the critical points of the CDW phase transition and the SC dome are also different for the two cases. The excitonic interaction appears to play an important role in the $P-T$ phase diagram of $1T-\mathrm{TiS}{\mathrm{e}}_2$, but not in the $x-T$ phase diagram.

The orbital degree of freedom of electrons greatly influences the physical properties of materials such as magnetic order and unconventional superconductivity. An orbital is a minimal unit of “shape,” and the orbital state can be unraveled by observing the spatial anisotropic distribution of electrons. However, it is difficult to experimentally extract the orbital information in a crystal because of various technical problems. Here, the $\mathrm{Ti}\text{−}3d$ orbital state in perovskite-type oxides $R\mathrm{Ti}{\mathrm{O}}_3$ ($R=\mathrm{Y}$, Sm, and La) is directly determined by a core differential Fourier synthesis (CDFS) method using synchrotron x-ray diffraction. The valence electron-density distribution, including information on the anisotropy and the hybridization between atomic orbitals, can be extracted from the CDFS analysis. Our study not only demonstrates the relationship between the magnetic- and orbital-ordered states called the Kugel-Khomskii model, but also provides a nontrivial picture of the orbital state reconstructed by the orbital hybridization.